1. Field of the Invention
The present invention relates generally to alternators, and more particularly to an alternator of the type which is used in a vehicle employing a gasoline engine or diesel engine as the main vehicle engine and which is driven by this main vehicle engine to provide a source of alternating current having a high degree of frequency accuracy.
2. Description of the Prior Art
In known alternators of the above type which are driven by internal combustion engines (hereinafter simply referred to as engines), a so-called mechanical coupling method is employed in which the output shaft of the engine and the rotor driving shaft of the alternator are rotated in direct drive or at any other gear ratio or through a velt pulley, and the engine is provided with an engine governor such as a mechanical governor and the accelerator operating lever linked to the throttle valve of the engine, the fuel control rack of the fuel injection pump or the like to regulate the number of revolutions of the engine.
A disadvantage of this conventional arrangement in which the engine is mechanically coupled to the alternator and the number of revolutions of the alternator is regulated by the engine governor is that no matter how the efficiency of the engine governor were improved, it would be extremely difficult to reduce the fluctuation of revolutions below 0.5 % due to causes such as a time delay between the feeding of fuel to the engine and the conversion of the fuel to rotating energy, and variation in the rotating torque generated in each combustion cycle.
However, due to the growth in recent years of the number of electronic devices of the type which are locked to the power supply frequency, such as, a video tape recorder in which the variation in the alternating current power supply frequency must be limited to less than 0.33 % of the preset value (e.g., less than 0.2 Hz for the 60 Hz power source), it is not infrequent that the frequency accuracy of the conventional power supply units is found to be insufficient as compared with that required for the installation of such electronic devices in a vehicle.
While another method has been proposed in which the output of an engine driven-alternator is converted into a direct current to charge the battery, and the direct current is also converted back into alternating current by a stationary inverter, it also has the disadvantages such as the problem of waveform distortion, deterioration in the efficiency of the system on the whole, and increased manufacturing costs.